Pipelines for transporting liquid or gaseous fluids, such as gas, water or oil pipelines, are generally constructed from lengths of metal pipe, generally of steel construction, which are welded together to form an assembled continuous pipeline which is laid in its final seat, generally consisting of a sufficiently deep trench, and then covered so that the pre-existing environment is restored and its subsequent utilization unhindered.
The assembled continuous pipeline is provided with protection against wet corrosion in that the environment in which the underground or immersed pipeline is located is very corrosive towards ferrous materials.
It is very important that the pipeline remains sound and well preserved for the entire duration of its technical life, not only because of its high construction cost but also, and in particular, because leakages of fluid must be prevented not only because of its economic value but also because it can be the cause of danger, pollution or serious disturbance. The protection generally adopted is in the form of two different types of protection which cooperate with each other, namely "passive" protection consisting of a covering which isolates the pipeline from the surrounding environment and "cathodic" protection by which it is given an electrical potential which inhibits the progress of possible electrochemical reactions which could attack the pipe metal until perforated.
The "passive" protection is generally formed by applying a continuous covering to the properly cleaned pipeline. This covering is of two main categories, the first of which comprises coatings of hydrocarbon materials such as asphalts, bitumens and greases generally applied hot in the form of a number of coats, with fibrous material reinforcements. The second category comprises coverings of synthetic polymer materials such as polyethylene, polyvinylchloride and epoxide polymers which are applied in the form of tapes wrapped spirally about the pipeline with their edges superimposed, or alternatively are applied by die-casting. Widely used protection and reinforcement materials include fibreglass tape, felt and cardboard, asbestos or other non-decaying fibrous material.
By itself, such protection is insufficient to permanently preserve a pipeline laid underground or immersed in water.
In this respect, the following facts must be considered:
no material is free of a certain porosity and permeability, even if perfectly applied, resulting in a certain though very slight passage of the chemical species responsible for the corrosive attack through the protective layer;
the sequence of operations involved in the preparation, covering, lifting, laying and burying of the pipeline can from the very beginning give rise to slight damage or imperfection in the applied covering, such defects then leading to future triggering of corrosion phenomena;
the hydrocarbon and polymer materials and their reinforcements have a very high but not absolute chemical and physical stability, especially where temperature or humidity variations occur;
natural phenomena such as earthquakes, landslips or alluvions, and accidental events can cause damage to the pipeline passive protection.
The "cathodic" protection provides protection for the pipeline at points in which porosity, damage or imperfect application of the covering have left the metal surface exposed to corrosive attack. A typical cathodic protection scheme is illustrated in FIG. 1. The pipeline 1 comprises the metal pipe 2 and its covering 3. It is segmented into discrete portions by isolating joints 4 which separate them electrically without interrupting continuity. The length of each portion is indicatively some kilometers, but this value can vary within very wide limits depending on the importance of the installation and its environment.
Of each pipeline portion, the metal part 2 to be protected is connected by the connector 5 to the negative pole of a direct current generator G, the positive pole of which is earthed through the connector 6, which is provided with a switch 7 and ammeter A, and then through a disperser electrode 8.
Said disperser electrode is generally constructed as one or more large-dimension cylindrical graphite elements and is positioned at a considerable depth and spacing so as to ensure that the electrical field which is established is correctly distributed along the entire pipeline portion. By virtue of the current fed to the pipeline, this latter assumes a negative potential with respect to its surrounding environment.
This electrical potential is measured by the voltmeter V which is connected by the connector 9 to the metal pipe 2 and by the connector 10 to a reference electrode 11 positioned in proximity to the outer part of the covering 3. It is apparent that the quantity of electric current required of the generator G to maintain the metal pipe 2 at a certain potential relative to the environment surrounding the pipeline is inversely proportional to the insulation provided by the covering 3 and directly proportional to the extent which the defects in, or the permeability of, said covering allow the electrochemical contact which generates corrosion.
The cathodic protection potentials are of the order of magnitude of one volt, and with efficient coverings the current required to maintain such potential difference relative to earth are not large. Direct measurement of the insulation from the ratio of the potential difference established in the pipeline with respect to a reference electrode to the imposed current is only significant in those cases in which the deterioration of the covering has reached such proportions that it cannot be remedied.
In this respect it should be noted that an exposed area of the order of one mm.sup.2 per m.sup.2 of covering is already significant from the corrosion aspect, but this would result in a change in potential (for equal imposed current) which would be practically unnoted by the measurement.
Many methods have been proposed in the known art for determining the insulation offered by the covering 3, but most of them can only be usefully used in the laboratory or, at the most, on site before laying the pipeline in the trench, and are therefore more suitable for preventive inspection by sampling than for effective field verification of an operating pipeline.
The only method for this verification which is currently of wide application is based on the measurement of the variation in the potential difference when the current fed by the generator G to the metal pipe is interrupted. This is illustrated in FIG. 1. The cathodic protection circuit shown in FIG. 1 is in operation and the generator G is feeding the current I.sub.reg required for the pipe 2 to assume the scheduled negative potential .DELTA.V.sub.reg necessary for its cathodic protection.
At time t, the switch 7 is opened to interrupt current passage, as indicated in diagram (A) of the FIG. 2. When the measurement has been made, the current feed I.sub.reg is then restored in accordance with the dashed line.
Because of the feed interruption, the potential difference decays rapidly from the value .DELTA.V.sub.reg. The first part of the potential drop .DELTA.V.sub.ohm is substantially simultaneous with the interruption in the current and corresponds to the "ohmic" potential drop due to the resistance offered by the covering 3. The second part .DELTA.V.sub.reat of the potential drop is slower, with asymptotic progress downwards, and is due to the reactive component of the circuit. The voltmeter V of FIG. 1, provided with a recorder REG, measures the pattern shown in diagram (B) of FIG. 2 in which the potential drop is clearly divided by a point of distinct tangency variation which separates the two components of the potential difference, allowing a reliable measurement of the value of .DELTA.V.sub.ohm which is proportional to the insulation of the covering 3, with consequent indication of its soundness or lack thereof.
In reality the described method is not free of considerable problem deriving from the actual need to interrupt the protection current in order to effect the measurement.
At time t, at which the protection action ceases, instantaneous currents begin to circulate in the pipeline due to different potential points along the pipeline which arise from local electrochemical reactions. These currents give rise to a potential .DELTA.V.sub.ec the pattern of which is shown indicatively in diagram (C) of FIG. 2, the voltmeter V in reality then measuring a potential value determined by the sum of diagrams (B) and (C) of FIG. 2 and shown in FIG. 2 as diagram (D) . The pattern of diagram (D) is such that it is very difficult to determine the ordinate value at the point in which the change of tangency separating the ohmic and reactive terms of the potential difference occurs.
Said instantaneous currents are the cause of large errors in determining the insulation resistance, in that although the imposed protection current is instantaneously nullified at the moment of interruption, it is precisely this interruption which generates said electrochemical currents which disturb the system to give rise to a disturbed pattern of the potential diagram of the type shown in FIG. 2 (D).
The nature and extent of these electrochemical currents are related to the heterogeneous state of the ground and to the active corrosion-producing chemical species present in it, such as different dissolved oxygen concentrations, pH variations, different global ion concentrations, different defect distributions along the pipeline, etc.
Under such conditions the potential drop consequent on zeroing the cathodic protection current does not assume a properly defined profile, and it is not possible to measure the value .DELTA.V.sub.ohm with sufficient reproducibility, as the point of tangency change cannot be properly identified even with sophisticated recording voltmeters.
To this phenomenon there is also added the fact that with the aforesaid method the current zeroing produced by real switches is often not instantaneous but progressive, even if rapid, with the result that the ohmic drop also has an inclined rather than vertical pattern, making it even more difficult to estimate with sufficient exactness the ordinate of the point of tangency change. Another important problem is the fact that during the measurement the pipe 2 is no longer cathodically protected and is exposed to corrosion. In practice, this method is used only during occasional inspection, and for long time periods when the state of the pipeline covering is not checked. In any event the method is unsuitable for continuous monitoring of the covering because the total time involved in testing would result in a considerable period during which the pipeline was not under cathodic protection. It does therefore not allow damage of accidental type to be indicated in sufficient time to take the necessary action.
The prior art has proposed, such as in U.S. Pat. No. 4,255,242, to determine the polarized potentials and the ohmic drops on cathodically, or anodically, protected materials, but disregarding the presence of protective coatings. According to such a procedure, waveforms are used which are not square, but are variously clipped sinusoidal waves. In such a case, the detection of the variations of the induced potential cannot be correlated to a value of impressed current and, moreover, it is not possible to separate the values to be attributed to the ohmic effects or the capacitive effects (or the reactive effects) which play a completely different role.
In the paper by Kasahara et al., on Materials Performance, Vol. 18, No. 3, pages 21-25 (1979), a method is disclosed for the simulated determination of the protection potential of a coated circuit, in correlation with the current pulses delivered to a sample fitted with a coating similar to that of the conduit and connected in parallel thereto, by measuring the potential induced into the sample.
Such a determination cannot be exploited to achieve any reliable information as to the condition of the coating on the conduit. Even if the sample should simulate with fidelity the state of the conduit at a certain instant of time, and give a substantially correct value of the induced potential, this data would not afford the solution of the technical problem posed herein, which is based on the "response" (in relationship with the ohmic component only of the potential included thereby) to the current modulation with a square wave, and to its trend with the lapse of time.